The present invention relates to variable displacement compressors suitable for automotive air conditioning systems.
Typically, variable displacement compressors are employed in automotive air conditioning systems. A typical variable displacement compressor has a housing that houses a crank chamber and supports a rotatable driving shaft. Cylinder bores extend through a cylinder block, which forms part of the housing. A piston is accommodated in each cylinder bore. A cam plate is supported to rotate integrally with the drive shaft, while inclining in the axial direction. The peripheral portion of the cam plate is connected to each piston. A displacement control valve adjusts the difference between the pressure of the crank chamber and the pressure acting on the pistons in the cylinder bores (hereafter referred to as the first differential pressure .DELTA.P1). The inclination of the cam plate with respect to a plane perpendicular to the drive shaft is altered in accordance with the first differential pressure .DELTA.P1 to vary the displacement of the compressor.
Typically, the variable displacement compressor is connected to an automotive engine by an electromagnetic clutch. The clutch is actuated to connect the engine to the compressor when activating the air conditioning system.
When the cam plate is arranged at a maximum inclination position to maximize displacement, a rise in the engine speed may rotate the drive shaft at a high speed. In such case, the compression load increases in a sudden manner. This increases the product of the pressure between contacting surfaces of moving parts and the velocity of the contacting moving parts (i.e., Pv value). As a result, the life of the moving parts and the compressor is shortened.
Such shortcomings have been overcome by de-actuating the electromagnetic clutch to stop operation of the compressor when the acceleration pedal is depressed to increase the engine speed and accelerate the vehicle. The electromagnetic clutch is de-actuated when parameters such as the engine speed, the intake air pressure, and the depression angle of the acceleration pedal, indicate acceleration. However, this increases fluctuations in the temperature of the air passing through an evaporator. As a result, warm air enters the passenger compartment, which may make the passenger compartment uncomfortable during acceleration. Additionally, the shifting of the electromagnetic clutch between actuated and de-actuated states produces torque shocks.
There are also vehicles that continue operation of the compressor during acceleration. However, this interferes with acceleration and lowers fuel efficiency.
Accordingly, U.S. Pat. No. 4,872,814 proposes a variable displacement compressor that overcomes these shortcomings. The structure of this compressor is similar to the compressor that employs the cam plate but has a mechanism that shifts the displacement from maximum to minimum when the rotating speed becomes too high. As shown in FIG. 22 herein, the displacement shifting mechanism includes a pressurizing passage 101 that connects a crank chamber with a discharge pressure region (e.g., discharge chamber). The pressurizing passage 101 has a port 104. A valve body 102 is arranged on the drive shaft 103 to rotate integrally with the drive shaft 103. The valve body 102 further moves relative to the drive shaft in a direction parallel to and perpendicular to the axis L of the drive shaft 103. Movement in these two directions causes the valve body 102 to open or close the port 104. Under normal conditions, the forces of the springs 105, 106 cause the valve body 102 to close the port 104.
The valve body 102 includes a weight 102a. If the engine speed N increases and causes the rotating speed of the drive shaft 103 to exceed a predetermined limit value Nc when the displacement of the compressor is large, centrifugal force is applied to the weight 102a, which rotates integrally with the drive shaft 103. This moves the valve body 102 in a radial direction to the axis L against the force of the spring 105 and opens the port 104. When the port 104 is opened, the pressure of the discharge pressure region is communicated to the crank chamber through the pressurizing passage 101. This increases the pressure of the crank chamber. Consequently, the first differential pressure .DELTA.P1 increases and decreases the displacement. Since this reduces the compression load, the application of excessive load on parts subject to friction is avoided.
If cooling of the condenser is insufficient when the displacement of the compressor is large, the pressure of the discharge pressure region becomes abnormally high. In such case, the pressure of the discharge pressure region that is communicated through the port 104 moves the valve body 102 in a direction parallel to axis L against the force of the spring 106 and opens the port 104. This communicates the pressure of the discharge pressure region to the crank chamber through the pressurizing passage 101 and increases the pressure of the crank chamber. As a result, the displacement decreases and reduces the compression load. This avoids the application of excessive load on parts subject to friction.
FIG. 23 is a graph illustrating the characteristics of the compressor of the '814 patent. Zone 109 (slanted lines) represents the range in which the rotating speed N exceeds the predetermined rotating speed limit value Nc of the drive shaft 103 (depicted by solid line 107) or in which the difference between the pressure of the discharge pressure region acting on the valve body 102 and the pressure of the crank pressure region (hereafter referred to as second differential pressure .DELTA.P2) exceeds a predetermined limit value .DELTA.Pc (depicted by solid line 108). That is, zone 109 indicates the range in which the displacement is forcibly decreased to reduce the compression load of the compressor (regardless of the demand for cooling).
However, the compressor of the '814 patent also has several shortcomings. First of all, the valve body 102, which functions as a centrifugal valve, causes imbalanced rotation of the drive shaft 103. Imbalanced rotation of the drive shaft 103 may hinder compression motion. This increases torque fluctuation and degrades the driving comfort of the vehicle.
In addition, the displacement is not decreased unless either the drive shaft rotating speed N exceeds the predetermined limit value Nc or the second differential pressure .DELTA.P2 exceeds the predetermined limit value .DELTA.Pc, even if the rotating speed N and the second differential pressure .DELTA.P2 are both close to the associated limit values Nc, .DELTA.Pc. Therefore, to avoid excessive wear of moving parts caused by friction, conditions such as those represented by a corner zone S (indicated by crossed lines), in which the rotating speed N and the second differential pressure .DELTA.P2 are both close to their limit values Nc, .DELTA.Pc must be avoided by lowering the limit values Nc, .DELTA.Pc, as depicted by broken lines 107, 108 in FIG. 23. However, this would lead to overprotection of the moving parts, especially when one of the lowered limit values Nc, .DELTA.Pc is exceeded, but the conditions are still outside the corner zone S. In such state, demands for cooling cannot be fulfilled in a satisfactory manner.